This invention relates to a pneumatic booster, and particularly to a pneumatic booster for use in a hydraulic braking system of a vehicle such as an automobile, wherein the pneumatic booster is interposed between a brake pedal and a hydraulic master cylinder for increasing the force applied on the brake pedal by utilizing vacuum pressure in an intake manifold of an engine or pressurized air of an engine driven air pump thereby transmitting the increased force to the master cylinder and generating hydraulic pressure in the master cylinder.
In a conventional pneumatic booster the boost ratio or the ratio between the output force and the input force is fixed. Thus, when the pneumatic booster is incorporated in a braking system of a truck, the braking effect attained on the vehicle will differ substantially between a heavy loaded condition and light loaded condition. In other words, it is required to increase the braking force or the braking pressure in the heavy loaded condition as compared with the light loaded condition with respect to the same brake applying force.